The present invention relates to a process for manufacturing rare earth-iron-boron permanent magnet alloy powders by the reduction/diffusion (R/D) method.
Alloys with rare earth metals as principal components are used as permanent magnet material, magnetostrictive material, optomagnetic recording material, hydrogen occlusion material and magnetic sensor.
Magnets made of these alloys, e.g. R-Fe-B permanent magnets, have good magnetic properties. Two processes are now in use for the manufacture of alloy powders for R-Fe-B permanent magnets, namely, the powder preparation method of powder metallurgy (P/M) processing and the reduction/diffusion method. In the powder metallurgy method, ingots of rare earth metals and alloying elements are melted using a high frequency melting furnace to form an alloy ingot which is subsequently crushed into powder. However, it is disadvantageons to make powders by crushing because the rare earth metals are easily oxidized during crushing and hence the quality of the alloy is adversely affected.
To eliminate this disadvantage, the reduction/diffusion method was developed. According to this method, a rare earth (R) metal alloy powder is prepared in the following manner. Starting materials consisting of a rare earth metal oxide, iron or cobalt powders and ferroboron powders are mixed with calcium as reducing agent. The mixture obtained is dry pressed and heated in an inert gas atmosphere or vacuum, so that the rare earth metal oxide is brought into contact with melted or vaporous calcium, for reduction. At the same time, the rare earth metal formed by reduction diffuses into the particles of ferroboron, iron or cobalt. Thus, a R-Fe-B alloy powder of uniform composition is obtained. The reaction product obtained is a mixture of CaO formed as a by-product, unreacted excess metallic calcium, and the desired R-Fe-B alloy powder. These components exist in the form of a sintered mass. When the mass is crushed and treated with water, CaO and metallic Ca react with water to form Ca(OH).sub.2, and the alloy powders can easily be separated from Ca(OH).sub.2. When immersed in water, the mass disintegrates in a short time, forming a slurry with Ca(OH).sub.2 being in the upper layer of the suspension which is subsequently removed. Residual Ca(OH).sub.2 is removed by washing the alloy with acetic acid. R-Fe-B alloy powders are thus obtained. Rare earth metal oxides are less costly than rare earth metals. Therefore, this method of manufacturing R-Fe-B alloy powders from RE oxides is more economical than the powder preparation method and is generally preferred.
The two key techniques for the reduction/diffusion method are the prevention of oxidation of the R-Fe-B alloy powders, and the complete removal of residual calcium. The process steps that embrace these two techniques are the steps of disintegration, deionized water washing and acid washing described hereinbefore.
The prior art relating to the manufacture of R-Fe-B alloy powders predominately relate to the reduction/diffusion process, and rarely the wet process. Most of the literature on the wet process relates to acetic acid washing, which can result in the oxidation of R-Fe-B alloy powders leading to the loss of significant amounts of rare earth metal and, consequently, diminution of the magnetic properties of the resulting alloy.
To overcome the drawbacks described above, Japanese patent 63-310906 discloses the addition of EDTA (ethylene diaminetetraacetic acid) as complexing agent and corrosion inhibitor. However, the effectiveness of acids such as EDTA in the removal of CaO is pH dependent. If the pH value deviates, the effectiveness will be materially affected and the R-Fe-B alloy powders can be oxidized. Besides, the linseed oil (linoleic oil), added to prevent the alloy powders from being oxidized during the wet process, will coagulate the powders and hamper the removal of CaO. In addition, NaOH and HNO.sub.3, as disclosed by the patent, are added during the wet process to break up the coagulation of the powders caused by the linseed oil and thus facilitate the separation. However, the use of NaOH, a strong base, and HNO.sub.3, a strong acid, will make pH control difficult and the heat of neutralization generated will facilitate oxidation of the rare earth metal. Thus, permanent magnets made from the alloy powders having reduced magnetic properties will be produced in accordance with the Japanese patent method.